In conventional xerography, a xerographic plate formed of a layer of photoconductive insulating material coated on a conductive surface is used for establishing and supporting electrostatic latent images. The xerographic plate is electrostatically charged uniformly over its surface and a light pattern corresponding to the image to be reporduced is projected on the plate to selectively dissipate the charge. The resultant latent electrostatic image is developed by powdering with toner particles which are electrostatically attracted to the xerographic plate in a pattern defined by the charge distribution of the electrostatic image. The powder image is thereafter contact transferred to a sheet of paper or other print-receiving medium. Because of interfering electrostatic lines of force established in the latent image, the field lines of force concentrate at the edges of the latent image resulting in reproductions having hollow centers and emphasized edges.
These problems are overcome in the apparatus and method described in the McFarlane U.S. Pat. Nos. 3,339,469 and 3,220,833 wherein the electrostatic latent image is first formed on a screen or interrupted grid of conductive material coated with an insulative photoconductive material. Developing ink toner may be, in one embodiment, clouded onto the electrostatic latent image by filtering the toner particles through the screen or grid mesh onto the charged boundaries of the interstices of the screen in accordance with the charge pattern. The developed toner image is thereafter transferred by projection of the particles along the lines of an overall applied electrostatic force field across a space to a print-receiving medium or other transfer medium. The expedience of noncontact printing, and fine control over the electrostatic latent image charge distribution provided by the interrupted surface of the screen which supports the image, overcome many of the problems of conventional xerography.
Improvements on the McFarlane system are described in the Pressman patent application Ser. No. 673,499 and the Pressman and Kittredge patent application Ser. No. 776,146.
According to these disclosures, there is provided in a preferred embodiment a multi-layered apertured screen including at least a conductive layer and an adjacent insulative layer on which an electrostatic latent image is formed for modulating the flow of charged toner particles or other printing particles directed through the apertures of the screen by an accelerating field. A two-layer screen construction permits the establishing of charge layers on opposed sides of an insulator ("double layer charge") for selectively producing overlapping lines of force or "fringing" fields within the apertures of the screen. The fringing fields extend into and are substantially confined to the apertures, since they are short range and extend no more than a few aperture diameters at full strength. Thus, lines of force generated by fringing fields extend within apertures of the screen and can be oriented to oppose the passage of charged particles, enhance the flow of charged particles, or be neutralized to have no effect on charged particles directed through the screen. Thus, the double layer charge can be selectively established across the fact of the screen to substantially completely block the passage of charged particles through certain apertures, enhance and accelerate the passage of charged particles through other apertures, and control the width and density of the particle stream through other apertures along a continuous range in between. A stream or flow of charged particles directed through the screen by an accelerating field is therefore modulated to provide a cross-sectional density variation at least substantially corresponding with the image to be reproduced. The modulated stream of toner particles or other charged particles is transferred by the overall applied electrostatic projection field across a gap to a print-receiving medium and the powder image is thereafter fixed according to known techniques.
In one approach for establishing the double charge layer electrostatic latent image across the screen, a substantially uniform charge distribution ("pre-image charge") may be initially established across both sides of the insulative layer, with opposite charges on the respective sides, to provide a substantially uniform array of fringing fields within the apertures. These fringing fields include blocking fields to block or partially block particles of a predetermined polarity. By use of photosensitive materials the charge distribution, and therefore the blocking fields, are "imagewise modified" or "imaged" i.e. selectively dissipated according to a light pattern projected on the screen which corresponds to the image to be reproduced. The fringing fields also may include enhancing fields which are established by additional techniques. The screen must generally be charged initially with blocking fields which block the passage of charged toner particles through the screen. The blocking fields are thereafter selectively neutralized according to the light pattern or reversed to enhancing fields according to additional procedures, and the resulting modulation of the flow of charged particles through the screen results in negative printing. In order to achieve positive printing, special contact charging techniques and other expedients are used.